The present invention relates generally to thermally protective and insulative covering for components, and more particularly to a novel layered covering structure for the thermal protection of heat sensitive components and cryogenic storage vessels against the effects of periodic exposure to alternate high and low radiant heat fluxes.
In the efficient operation of spacecraft, such as orbiting satellites or the like, thermal insulation for heat sensitive components may be essential. The requirement for suitable thermal protection may be particularly important in orbiting satellites or other vehicles subjected to the extreme hot and cold of an orbit which cyclically exposes the vehicle to the sun and to the umbra of the earth.
Conventional thermal insulative coverings may consist of a multilayer structure of alternate layers of metal foil and ceramic or fibrous spacers. This structure is characterized by low thermal capacity and high conductive/radiative resistance, and a very low effective thermal conductivity. For applications involving unsteady or periodic high and low heat transfer, such as that experienced by an orbiting vehicle, the heat leak through the insulative covering structure will be substantially affected not only by the thermal conductivity value characterizing the insulative structure, but also on its thermal capacity. Conventional thermal insulation coverings used, for example, to insulate tanks containing cryogens as propellant reactants aboard an orbiting vehicle, allow approximately 0.1% per day loss of cryogen by reason of heat leak and resultant boil-off loss.
The novel insulative covering of the present invention provides substantially improved heat leak characteristics under conditions of periodic exposure to and removal of a heat flux thereon, of particular utility for the protection of heat sensitive systems or cryogen tankage aboard orbiting vehicles. The structure of the covering comprising the invention includes a plurality of alternate layers of metal foil and heat absorbing phase change material as the outer layers of the structure. The structure is therefore characterized by optimum thermal conductivity to thermal capacity ratio (i.e., thermal diffusivity) to minimize the heat transfer through the insulation during periods of exposure to high heat flux, by absorption of heat in the outer layers comprising the phase change material. The stored heat may be reradiated to deep space during periods of no (or low) heat flux exposure. Use of the present invention for orbiting vehicle thermal control or cryogen storage tank protection allows reduction in weight of required insulation and, most importantly, substantial reduction in heat leak to the protected components. The reduction in cryogen boil-off may allow significant extension of vehicle orbit lifetime.
It is therefore a principal object of the present invention to provide an improved thermal insulation structure.
It is a further object of the invention to provide an improved insulation structure for the protection of heat sensitive components exposed alternately to high and low heat flux levels.
These and other objects of the present invention will become apparent as the detailed description of representative embodiments proceeds.